Wind exists in different speeds and intensities and may vary significantly over time. When people wear hearing aids in windy environments, the action of the wind directly on the hearing aid and on objects in its immediate vicinity can cause a variety of undesirable audible effects. These effects are usually referred to as wind noise. Wind noise is a severe problem for users of hearing aids. When wind noise levels are low or medium, wind noise can mask some speech signals and the hearing aid user usually experiences decreased speech discrimination. When the wind noise levels are high, the noise level in the hearing aid can be very high, possibly in excess of 100 dB SPL. In the worst case, wind can saturate the microphone, thereby causing extremely high noise levels and discomfort for the hearing aid user. Users therefore often switch their device off in windy conditions, since in windy surroundings acoustical perception with the hearing device switched on may become worse than if the hearing device is switched off.
It is known to counteract wind noise by mechanical constructional measures. Such measures alone, however, cannot usually eliminate wind noise to a satisfying degree.
Therefore, wind noise problems have been studied and many advanced noise detection and noise cancellation technologies have been implemented in digital hearing aids to attempt to reduce the detrimental effects of wind on hearing instrument performance.
Current wind noise canceling technologies suppress wind noise with high-pass filters or subtract an estimate of the wind noise from the noisy signal. Regardless of the method, effective wind noise reduction can be achieved only if the presence of wind noise can be reliably and consistently detected.
Unfortunately, wind noise exhibits properties and features also common to other noise signals encountered in daily life. Also, depending on wind speed, direction of the wind with respect to the device, hair length of the individual, mechanical obstructions like hats and other factors, magnitude and spectral content of wind noise vary significantly. For these reasons it is often difficult to uniquely classify the presence of wind noise and extract it from other environmental noises.
However, wind noise does also have several unique characteristics that facilitate its detection. Wind noise predominantly is a low-frequency phenomenon. Many of the available wind noise detection technologies make use of the low correlation between two spatially separated microphones or make use of the unique spectral patterns exhibited by wind noise.
A known wind noise detection method detects wind noise by computing the correlation between signals produced by two microphones, as disclosed in US2002/037088. A low correlation between the outputs from two different microphones can at times be applied to reliably detect the presence of wind noise. However, the correlation of wind noise created at different sources differs. Turbulence created at microphone ports causes signals with a low correlation. On the other hand, when turbulence is created by an object or obstruction in the vicinity of the microphone openings, the resulting wind noise signals at the microphones may be highly correlated.
A second wind noise detection technique is based on the signal from a single micro-phone. This method makes use of several well know wind noise properties: high magnitudes low auto-correlation, and energy content at very low frequencies. Such a method is disclosed in EP 1 339 256. In a further development, also disclosed in EP 1 339 256, pitch filtering and nonlinear filtering have been developed to minimize the attenuation of the speech target signal.
As to wind noise reduction, a wind noise reduction technique, disclosed in US2002/037088 for hearing devices with more than one microphone, is to switch the hearing aid from a two microphone directional, or beamforming, mode to a single microphone or omnidirectional mode (sometimes referred to as omni mode) when wind noise is detected. This technique may be combined by the mentioned approach of applying a high-pass filter when switching from the directional to the omnidirectional mode.
Alternatively, WO 03/059010 discloses a method that uses two omni microphones in a hearing aid for the purpose of achieving a wind noise insensitive hearing aid. This disclosure describes the use of two microphones with different wind noise sensitivities. When wind noise is detected, the signal from the microphone with the lower wind noise sensitivity is used as the hearing aid input signal.
In a single microphone hearing device, wind noise reduction may be achieved by reducing the low frequency gain in the frequency domain or by applying a highpass filter in the time domain, as disclosed in EP 1 339 256.
It is an object of the present invention to provide methods and devices that overcome disadvantages of prior art wind noise detection and reduction approaches and which especially should be suited also for relatively high level wind noise. The methods should be computationally not expensive, so that they may be implemented also in hearing devices with limited processing power. Preferably, the methods should not be dependent on the signal correlation as a major indicator for the presence of wind noise and therefore, in the case of more than one microphone, be equally suited for wind noise caused at the microphone ports and for the wind noise caused by other objects.